In general, in the mobile communication industry with regard to mobile phones, for example, a reduction in size and profile and a low transmission loss of a mobile communication module substrate have been required in accordance with a frequency increase in a communication frequency band. Patent Document 1 (identified below) proposes formation of an air layer, which is a low dielectric material, as a device that realizes a low transmission loss in a ceramic multilayer substrate.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-304064.
As disclosed in Patent Document 1, a ceramic multilayer substrate including an air layer exhibits low strength against impact and, therefore, has a problem in terms of practical reliability. In addition, in the method for manufacturing a substrate having the above-described configuration, an unsintered ceramic layer is removed after firing the substrate such that the resulting portion is made to be an air layer. However, a reaction layer having some thickness is present at the interface between the unsintered ceramic layer and a dense sintered ceramic layer, and it is difficult to remove the reaction layer. Therefore, the substrate has to include the air layer having a thickness of as much as 300 μm or more. Consequently, such an existing design does not sufficiently address a requirement to reduce size and profile for a mobile phone.